This study experimentally investigates the influence of sand-to-cement ratios on the delayed ettringite formation (DEF) in low-alkali Portland cement paste and mortar. An experimental matrix and conditions in terms of material properties and thermal cycling to accelerate DEF have been defined. The research assesses the length variations and pore structure evolution of cement paste and various mortar compositions. Qualitative pore analysis reveals that delayed ettringite could initially form in large pores [0.25–1.0 µm], such as the interfacial transition zone (ITZ) surrounding aggregates. Furthermore, the higher the amount of paste matrix, the larger the expansion. Nevertheless, the volume of ITZ, determined by mortar composition, significantly affects the expansion rate. Hardened cement paste is characterized by a large volume of pores below 250 nm and low precipitation rate of delayed ettringite. However, a higher rate was observed in mortar samples with fine aggregates (0.1–0.5 mm) with typical pore sizes larger than 250 nm that represent the regions around ITZs. The findings from this study emphasize the importance of ITZ content in influencing the DEF expansion evolution and magnitude.